The earth’s atmosphere has warmed more slowly over the last decade or so than in previous years – and a big question in climate science right now is why.

A new paper links the so-called hiatus to natural changes in the climate, saying it’s all down to what’s going on in the tropical Pacific Ocean. But we should expect faster warming to make a comeback, the authors warn.

According to a new paper in Nature, understanding natural changes in the Pacific Ocean is key to finding out what’s causing the “hiatus” and how long it’s likely to stick around.

Pause for thought

When scientists talk about what’s causing the slowdown in surface temperature rise, a couple of explanations usually come up. As the authors explain in the new paper:

“Two schools of thought exist regarding the cause of this hiatus in global warming: one suggests a slowdown in radiative forcing … and the other considers the hiatus to be part of natural variability.”

When volcanoes erupt they spit reflective particles into the atmosphere. One suggestion is that an increase in these particles together with a dip in the amount of solar energy reaching earth could be contributing to less-than-expected warming. That’s what’s meant by a change in radiative forcing.

But the new paper sits squarely in the second camp, saying the “hiatus” is part of natural climate variability. It’s all to do with sea surface temperatures in the tropical Pacific Ocean cycling between warm and cool phases, the paper says.

A good match

The paper looks at the period 1970 to 2012, which includes the current “hiatus” and a period of faster global warming between the 1970s and the late 1990s.

When the scientists included measurements of Pacific sea surface temperature over the 42-year period, they found their model reproduced recent global temperatures much better than when they only included changes in radiative forcing.

This shows the tropical Pacific plays an important role in climate, despite the fact it only accounts for 8.2 per cent of the earth’s surface, the authors say.

This graph shows the good match between model temperatures in the last few decades (in red) and measured temperatures (in black). Just accounting for radiative changes doesn’t reproduce the recent surface warming slowdown (in purple).

The scientists found the recent period of slower surface warming coincided with lower than usual sea surface temperatures in the tropical Pacific, caused by a tendency towards the cool phase of a natural cycle, known as La NiÃ±a. The paper explains:

“Our results show that the current hiatus is part of natural climate variability, tied specifically to La-NiÃ±a-like decadal cooling â?¦ For the recent decade, the decrease in tropical Pacific sea surface temperature has lowered the global temperature by about 0.15 degrees Celsius compared to the 1990s”.

In contrast, a tendency towards the opposite warm phase of the Pacific cycle, known as El NiÃ±o, caused global temperatures to rise much faster between the 1970s and the late 1990s.

Growing evidence

This may all sound familiar – it’s not the first time scientists have proposed a link between the “hiatus” and Pacific surface temperatures. Lead author on a recent paper in the Journal of Climate, Professor Gerard Meehl, tells us:

“This paper basically confirms, with a novel methodology, what we originally documented in our Nature Climate Change paper in 2011 and followed up with in our Journal of Climate paper”.

But Meehl adds, the new paper only tells half the story. For example, it doesn’t address why the tropical Pacific ocean shifts from cool to warm phases, or where the missing heat is ending up. He tells us:

“We went beyond [the new paper] to show that when the tropical Pacific was cool for a decade … more heat is mixed into the deeper ocean, something the new paper doesn’t address.”

Ocean temperature data – not part of the new study –indicatesthat at the same time surface temperature rise has slowed over the past decade or so, the rate at which the deep oceans take up heat has accelerated. The graph below is from the first of three special Met Office reports last month.

“The [new] paper has captured some essential parts of the cause of the apparent hiatus in global mean surface temperatures â?¦ However, it does not deal with why the sea temperatures have changed as observed”.

The earlier paper by Meehl, Trenberth and colleagues suggests a reason why the deep ocean shifts between taking up more or less heat – a natural cycle in the ocean known as the Interdecadal Pacific Oscillation. We wrote more about the paper, here.

Future warming

According to the new paper, the link with natural variability suggests the slower surface warming we’re seeing is temporary and “global warming will return when the tropical Pacific swings back to a warm state.”

Though similar “hiatus” events may occur in the future, temperatures will only be affected in the short term, the paper adds. As the Met Office said in another special report last month:

“[T]he heat in the ocean is merely being rearranged; it is effectively ‘hidden’ from the surface only to re-emerge at some later date.”

When looking at how global temperatures have changed, it’s easy to focus on the atmosphere because that’s where most measurements are made. But this paper adds to growing evidence that we need to look to the deep ocean to really take earth’s temperature.